The present invention relates to a knock detecting apparatus suitable for use with the ignition timing control apparatus of internal combustion engines, which functions so that the occurrence of knock is detected from the vibrations emitted to the outside of the engine cylinder due to the pressure therein and the ignition timing of the engine is adjusted so as to obtain the desired knock intensity.
It is well known in the art that there is a close correlation between the ignition timing and the cylinder pressure. When the mixture is exploded, in the absence of any knock there will be no superposition on the cylinder pressure of higher harmonic components (usually those frequency components in the ranges between 5 and 10 kHz and between 11 and 13 kHz or the components in the frequency bands which are determined by the bore diameter of the engine cylinders and the velocity of sound of the combustion and they are produced as the result of intermittent and rapid combustion). On the contrary, when the engine starts knocking, high frequency components start to be superposed on the cylinder pressure at around its peak value and this has the effect of causing emission of vibrations or sound to the outside of the cylinder. A close examination of the pressure signals produced in the cylinders or the variations or sound emitted to the outside of the cylinders will show that the beginning of knock (trace knock) occurs at an engine crank angle at which the cylinder pressure attains the maximum value and the knock is gradually increased (to light knock and heavy knock) causing the resulting higher harmonic components to begin to superpose considerably on the cylinder pressure at an earlier crank angle than that corresponding to the maximum cylinder pressure value (or on the ignition side). Thus, the engine efficiency will be increased greatly if the vibrations or sound emitted by knocking to the outside of the cylinders is detected with a high degree of accuracy and fed back to control the ignition timing. However, presently no detecting apparatus has been proposed which is capable of detecting the presence of knock or feedback factor with improved accuracy and which is also operable stable under severe environmental conditions required for vehicles.
Prior art detecting apparatus of the above type include those which employ the ordinary piezoelectric type accelerometers and resonant type detecting apparatus which has been investigated by the inventors. The ordinary accelerometer type is disadvantageous in that its resonant frequency is above the frequency band in which knock occurs and the detecting characteristic becomes flat for the frequencies below the knocking frequency (this type is hereinafter referred to as a nonresonant type). The resonant type has a resonant frequency within the knocking frequency band and thus it has a high sensitivity to the frequencies around the resonant frequency as compared with the other frequencies. However, in view of the nature of resonance, any attempt to increase the resonance sharpness Q inevitably tends to decrease the range of detectable frequencies.
Another disadvantage of the nonresonant type is that the S/N ratio of the apparatus is deteriorated by engine vibration noise so that even if the range of frequency detection is increased from the principal point of view, the detection of knock at high engine speeds will be made practically impossible due to such noise. On the other hand, the resonant type is excellent in both S/N ratio and sensitivity and thus its knock detecting capacity is much improved as compared with that of the nonresonant type. However, due to its high Q value and reduced frequency detection range, it has been the tendency of the resonant type to find it increasingly difficult to detect the occurrence of knocking if the knocking frequency distribution varies with the progress of combustion. This means that while the occurrence of knock is distributed over a wide frequency range, the resonant type detecting apparatus cannot ensure a uniform sensitivity characteristic (or a trapezoidal characteristic) in the frequency band and thus it is capable of detecting only a relatively narrow range of frequencies. While this detecting characteristic can still be said to be excellent as compared with that of the nonresonant type, it appears that if the knock detecting accuracy is to be improved further, it is desirable to ensure a trapezoidal sensitivity characteristic having a wide detection range even if the sensitivity of the apparatus is deteriorated as the result of the reduced Q.